ANALYTICAL ABSTRACTS. 97 



underlying strata in proportion to their weight. Such folds may be called 

 original." The Pottsville, Mahanoy, Shamokin and Wyoming coal basins of 

 Pennsylvania belong to this class. 



Experiments have recently been carried' on in the office of the United 

 States Geological Survey reproducing the different forms of folding. The 

 experiments differed from other experiments in that i) the materials used to 

 simulate the stratified rocks varied in consistency from brittle to plastic, 

 according to the depth at which deformation is supposed to take ]:)lace ; 2) 

 the compression was exerted under a movable load representing the weight 

 of superincumbent strata ; 3) the strata rested on a yielding base to simulate 

 the condition of support of any arc of the earth's crust. The following are 

 the conclusions from the experiments : 



1. "When a thrust tangentially affects a stratified mass, it is transmitted 

 in the direction of the strata, and by each stratum according to its inflexi- 

 bility. At any bend the force is resolved into components, one radial, the 

 other tangential to the dip beyond the bend ; the radial component, if 

 directed downward, tends to depress the stratum and displace its support. 



2. "A thrust so resolved can only raise an anticline or arch which is 

 strong enough to sustain the load lifted by its development ; such an arch 

 may be called competent ; and since strength is a function of the proportions 

 of a structure, it follows that, for a given stratum, the size of a competent 

 anticline will vary inversely as the load ; or for a given load the size will 

 vary as the thickness of the effective stratum. 



3. "The superincumbent load borne by a competent anticline is trans- 

 ferred to the supports of the arch at the points of inflection of the limbs. 



4. "When a competent arch is raised by thrust from one side, the load 

 transferred may so depress the resulting syncline further from the force that 

 an initial dip will be produced in otherwise undisturbed strata; this dip will 

 rise to a bend from which a new anticline may be developed. This anticline 

 is a result of the first, and may be called 'subsequent' in distinction to orig- 

 inal folds. Since subsequent folds are simply competent structures, their 

 size will be determined by conditions of thickness and load, and for like con- 

 ditions they should be equal ; and they must, in consequence of conditions of 

 development, be parallel to the original fold and to each other. An example 

 of an original fold with its subsequent anticlines is the Nittany arch and the 

 group of parallel anticlines which lie southeast of it, extending northeast 

 from the Broad Top basin." C. E. P. 



The Catskill Delta in the Post- Glacial Hudson Estuary. By William 

 Morris Davis. (From the Proceedings of the Boston Society of 

 Natural History, Vol. XXV., 1891). 



The post -Tertiary trenches of the Hudson and its tributaries are in the 

 main filled with clay beds, which, covered bv a thin deposit of sand, rise in 



